1. Field of the Invention
This invention relates to new polydiorganosiloxane-containing compositions that are curable with radiation; to a method of preparing a coated substrate using said compositions and to the coated substrate produced thereby.
In particular, this invention relates to certain compositions comprising mercaptoalkyl-containing polydiorganosiloxane and vinylic monomer which, when applied to a substrate such as paper and exposed to energetic radiation such as ultraviolet light, will cure rapidly to produce a coating, adhered to the substrate, that provides controllably variable release of adhesives.
2. Description of the Prior Art
Polyorganosiloxanes have been used for a long time to coat solid substrates, such as paper, to improve the release of adhesive materials therefrom. As the adhesive properties and holding power of adhesive materials have improved, the need for improved release coatings has increased. Improved release coatings have also been required by other factors such as increasing environmental concerns, a desire for faster coating and curing processes, and a need to lower process energy costs.
Solventless, radiation-curable coatings having premium release of aggressive acrylic adhesives have been disclosed by Bokerman, et al. in U.S. Application Ser. No. 663,324, titled "Radiation-Curable Mercaptoalkyl Vinyl Polydiorganosiloxanes", filed on Mar. 3, 1976 and assigned to the assignee of this application. The coatings of Bokerman, et al. are curable with radiation without the aid of a photosensitizer; however, more rapid cure rates are obtained if a photosensitizer, such as benzophenone, is used. Unfortunately, certain photosensitizers are sparingly soluble in mercaptoalkyl-containing polydiorganosiloxanes, and are therefore of limited utility. Solventless, radiation-curable compositions wherein larger, more effective amounts of certain photosensitizers can be dissolved are desirable.
Compositions have also been sought that would provide a controllably variable release of adhesives. Northrup, et al., U.S. Pat. No. 3,328,482, claim an intimate physical mixture of certain polyorganosiloxanes which are curable to an elastomer by a technique that does not use free radicals and a polyisobutylene having a molecular weight of over 400. The compositions of Northrup, et al. are not curable by radiation, but, rather, by the usual moisture-curing processes which are relatively slow or by heat-curing processess which are energy-intensive. Mestetsky, in U.S. Pat. No. 3,770,687 claims a controlled self-release coating composition comprising an organic solvent solution of any wellknown organosiloxane release agent and certain organic polymers. However, the use of organic solvents in coatings is environmentally undesirable. A radiation-curable organosiloxane coating that possesses controllably variable release over a wide range of release values and is free of solvent is also desired.
The reaction of a diorganopolysiloxane with a polymerizable olefin to produce a hydroxyl-terminated grafted polydiorganosiloxane is described by Neuroth in U.S. Pat. No. 3,436,252. The resulting composition, when used to provide release characteristics to paper, requires a curing agent and a solvent and is not radiation-curable.
The reaction of mercaptoalkyl-containing silanes, siloxanes and polysiloxanes with vinylic materials is not new. Cekada, et al. in U.S. Pat. No. 3,532,729 claims a method comprising preparing an emulsion of a siloxane containing at least one percent by weight of units of the general formula ##STR1## wherein A is an alkylene or arylene radical, R' is a methyl or phenyl radical and n has a value of from 0 to 2 and thereafter adding to the emulsion a non-polymerizable olefin or a polymerizable olefin and causing the addition of the non-polymerizable olefin to the siloxane by free radical catalysis or causing the polymerization of the polymerizable olefin and its addition to the siloxane by free radical catalysis. The emulsions of Cekada, et al. are relatively rich in silsesquioxane and are useful as paint additives when the olefin is acrylate, as molding compositions when the olefin is styrene, as additives to impart thermal stability and as textile treating agents. However, Cekada, et al. do not disclose or suggest a solventless composition comprising a mercaptoalkyl-substituted polydiorganosiloxane and a vinylic monomer that cures to a coating that has controllably variable release of adhesives.
The free-radical polymerization of an organic vinylic monomer, in the presence of a polyorganosiloxane, is disclosed by Saam, U.S. Pat. No. 3,686,356, and by Fiedler, U.S. Pat. No. 3,923,923. However, these compositions which are at least 40 weight percent organic vinylic monomer, have the general characteristics of organic vinylic thermoplastics.
The terms "controlled release" and "controllably variable release" as used herein mean that the force needed to remove adhesive from the surface of the cured compositions of this invention, under the applying, curing and measuring conditions hereinafter described, can be varied, i.e. increased or decreased, in incremental fashion over a wide range of values by controlling the relative amount of vinylic monomer in the composition to be cured.
A coating is considered to provide premium release if a force of not more than 100 grams per inch (38.61 newtons per meter), as measured by the method hereinafter described, is needed to remove the adhesive from the coating. An aggressive adhesive is a material that requires a force of at least approximately 460 newtons/meter (N/m) to remove the adhesive from a stainless steel surface using said method of measuring.